1. Field of the Invention
The present invention relates to spread spectrum transmitters and receivers in general and in particular to a method and apparatus comprising a spread spectrum transmitter having a carrier signal provided by a voltage controlled oscillator, the frequency of which is modulated by the output from a correlation code and data generator, and a spread spectrum receiver having separate channels for, respectively, locking the receiver to the transmitter and recovering data.
2. Description of the Related Art
In spread spectrum communication systems, a carrier signal and the information/data impressed on the carrier signal is spread over a wide range of frequencies. To enable multiple transmitters and receivers to operate over the same frequency band with a minimum of interference, it has been the practice to modulate the carrier in the transmitter with a code, e.g. pseudorandom binary code, and to demodulate the carrier in the receiver with the same code and thus lock the receiver to the transmitter using combined code shifting and crosscorrelation techniques. Thereafter, data which has also been impressed on the carrier can be recovered.
Heretofore, a number of problems have been encountered in spread spectrum transmitters. For example, when a transmitter is first turned on, the output frequency of its local oscillator typically changes over a wide range for a period of time until the oscillator comes up to its center frequency or until a voltage controlled oscillator (VCO) in a phase-locked loop (PLL) in which a crystal serves as a reference can be locked to the crystal frequency. This time can be as long as 20 milliseconds (msec.) in many systems. In a spread spectrum system this change in carrier frequency can result in interference with other narrowband receivers and be a source of interfering noise to a receiver which may be receiving data at the time.
In a typical conventional direct sequence spread spectrum system, a VCO in a PLL as described above or a standard multiplier chain is used to create a carrier signal having a nominal center frequency, e.g. 915 MHz, in a predetermined frequency range, e.g. 902 MHz to 928 MHz. This single frequency is then mixed with a correlation code from a correlation code generator and with data from a data generator. The mixing of the correlation code with the carrier signal can involve amplitude modulation, phase modulation or frequency modulation. The data may be mixed with the carrier signal either before or after the correlation code is mixed therewith.
The mixing of the correlation code and data with the carrier signal has a number of disadvantages. For example, it requires the use of mixers which tend to be expensive electronic components. Also, the mixing of the correlation code and data with the carrier signal causes sidelobes which can lead to false code signal synchronization, i.e. false receiver locks, and wasted energy.
Another problem encountered in prior known spread spectrum transmitters which use a VCO for generating a carrier signal is that certain components in the carrier modulation circuits in different transmitters can produce unpredictable deviations in the carrier frequency for given voltages applied to the varactor, thus requiring expensive and time consuming tuning procedures to enable multiple transmitters to communicate with a given receiver.
Known spread spectrum receivers also have problems. For example, conventional spread spectrum receivers typically employ combined locking and data recovery circuits and techniques which can be susceptible to interference, i.e. undesirable large numbers of false locks, and slow locks.